1. Field of the Invention
The present invention relates generally to multimedia transmission systems and more particularly to systems and methods for transmitting high definition digital video and standard definition analog video over a single cable.
2. Description of Related Art
With the advent of digital broadcast television and streaming video technologies various video cameras, monitors and video recorders have become available with enhanced resolution and advanced features. Closed circuit television (CCTV) systems now offer high definition video outputs and compressed digital video signals for use in applications such as premises surveillance, access control and remote monitoring of facilities. However, legacy systems remain in place and standard definition analog video signals are in widespread use and will continue to be used during the transition to all-digital, high-definition systems. In particular, coaxial cable (coax) has been extensively deployed to carry signals from analog CCTV cameras to monitoring stations. Also, some deployed CCTV cameras transmit compressed digital video signals over local area networks, and these cameras may use the Internet Protocol (IP) as a communications means for transmitting the compressed video signal over category 5 (CAT5) twisted pair cable.
FIG. 1 illustrates a conventional system using coax cable 14 to carry standard definition analog video. A basic analog camera 10 typically generates a composite video baseband signal (CVBS) that can be transmitted up to 300 meters or more using Coax 14. The CVBS signal is commonly provided to a video recording system 18 which often comprises a digital video recorder (DVR) that digitizes the CVBS signal and records it. A conventional monitor or display device 16 may be connected directly to coax 14 to display live standard definition video and to the DVR 18 for playback of recorded video. The standard definition (SD) video typically has a resolution of 720×480 pixels.
FIG. 2 illustrates conventional approaches to transmitting high definition (HD) video (1920×1080 pixels) in conventional systems. An IP based, HD camera 20 generates a compressed digital HD video signal which is transmitted using 100 Mbps Ethernet over standard CATS twisted pair cable 24 for distances of up to 100 meters. The signal is received by a host processor and DVR 28. The HD video can be viewed live and also recorded for non-real time playback. The use of IP networking to enable the camera 20 to transmit digital video allows these systems to add some upstream communications from monitor-side to camera-side; this upstream communications consisting of camera control and audio signals. It should be noted that for the live video, noticeable delay may occur due to latency in the IP network and due to the time needed for a processor (e.g. in host DVR 28) to reconstruct the compressed digital video. However, the use of IP networking enables the use of networking tools, including routers, to combining traffic to or from multiple cameras and/or DVR recording and monitoring devices in different network connected locations.
Framing in Digital Communications Systems
Almost all digital data streams have some sort of frame structure such that the data is organized into uniformly sized groups of bits or bytes. Any system that uses block based forward error correction (FEC) will have frames organized around the error correction code word size. Also, if the system uses interleaving to combat impulse noise, the frame structure will be arranged with the interleaver parameters in mind. If the system uses data randomization to achieve a flat spectrum, the pseudo-random sequence utilized may be synchronized to the frame structure, restarting at the beginning of each frame.
For an RF digital communications system, a receiver must typically first achieve carrier and symbol clock synchronization and equalization. It can then recover the transmitted data. But, to make sense of this incoming data stream, the receiver must also synchronize to the frame structure. In other words, the receiver must know where the error correction code words start and end. It also must be able to synchronize receiver modules such as the deinterleaver to match the interleaver operation of the transmitter so that the resultant deinterleaved bits or bytes are correctly ordered, and the de-randomizer to match the starting point of the pseudo-random sequence used in the transmitter to flatten the spectrum.
Conventional systems often provide for receiver frame synchronization by appending a known pattern of symbols of a fixed length at the beginning or end of the frame. This same pattern repeats every frame, and it often consists of a 2 level (i.e. binary) pseudo-random sequence with favorable auto-correlation properties. This means that while the auto-correlation of the sequence with itself at zero offset yields a large value, if the offset is non-zero the correlation value (side-lobe) is very small. Also the correlation for this frame sync sequence with random symbols will yield a small value. Therefore, if the receiver executes a correlation of the incoming symbols with a stored version of the frame sync pattern, it should expect to yield a large value only at the exact start of each frame. The receiver can then easily determine the starting point of each frame.
There can be several modes of operation for the communication system. The modes can include a variety of combinations of symbol constellations, trellis codes, and interleave patterns. The receiver must have knowledge of the mode in order to successfully recover the transmitted data. This can be achieved by adding additional mode symbols to the frame sync pattern. These mode symbols can be reliably received by using correlation methods since they are sent repeatedly every frame. They can be made even more robust by encoding them using a block code.